The use of optical fibers in communications is growing at an unprecedented rate. Low loss optical fibers which are produced by any one of several techniques may be assembled into ribbons which are then assembled into cables, or stranded into cables, or they may be enclosed singularly in a jacket and used in various ways in a central office, for example.
In order to assure that the low loss fibers which are produced today are not diminished in their effectiveness in systems, the fibers must be connected through intermateable connectors which preserve those low losses. For fiber ribbons, connectors comprise grooved chips which hold a plurality of fibers of one ribbon in alignment with fibers of another ribbon. Such a connector is shown for example in U.S. Pat. No. 3,864,018 which issued on Feb. 4, 1975 in the name of C. M. Miller.
For single fiber cables, connections may be made through a connector which is referred to as a biconic connector. See U.S. Pat. Nos. 4,107,242 and 4,512,630 which issued on Aug. 15, 1978 and Apr. 23, 1985, in the name of P. K. Runge. That connector includes a housing in which is mounted a biconic alignment sleeve. The sleeve includes two truncated, conically shaped cavities which communicate with each other through a common plane which has the least diameter of each cavity. Each of two fibers to be connected is terminated with a plug comprising a primary pedestal or truncated, conically shaped end portion which is adapted to be received in one of the cavities of the sleeve. The conically shaped surfaces of the plug and of the sleeve serve as alignment surfaces. The fiber extends through a passageway in the plug and has an end which terminates in a secondary pedestal of the plug. Generally, a plug is molded about an end portion of an optical fiber; however there is a demand for plugs having passageways molded therein for the field termination of optical fibers. A cylindrically shaped portion of the plug is connected to the truncated end. The plug is urged into seated engagement with the wall defining the cavity in which it is received.
Minimal loss between the connected fibers is achieved when the cores of fibers which are terminated by the plugs are aligned coaxially and when the longitudinal offset along the axes of the plugs is zero and fiber end faces, each of which is planar, contact in a common plane. Considering the size of the fibers, for example one with a core diameter of 8 microns and a cladding diameter of 125 microns, the task of providing conical plug and sleeve surfaces in order to meet alignment and end separation requirements is a formidable one. Generally, the plugs are molded from a transfer molding grade epoxy composition material. Although the surface tolerances which are achieved when molding the alignment sleeves and conic tapers are excellent, they are not sufficient to achieve consistently the desired alignment and end separation.
Problems arise because the opening in the end face of the pedestal and hence the fiber core may not be centered with respect to the axis of revolution of the truncated, conically shaped end portion. The axis of revolution of the conically shaped end portion may also be referred to as the conical axis. As a result, the cores of the fibers terminated by two plugs held in the sleeve may have sufficient transverse or lateral offset to affect adversely the transmission of signals. This problem has been overcome by the methods and apparatus of copending, commonly assigned application Ser. No. 802,500 filed of even date herewith in the names of R. P. Lyons et al. Also the centroidal axis of the end portion of the core of the fiber disposed in the passageway may not be coincident with to the axis of revolution of the conically shaped end portion of the plug. Consequently, the light emitted from one fiber may not be parallel to the axis of the receiving fiber. This problem which is referred to as angular offset may also occur when the plug is molded about an end portion of an optical fiber. The angle between the fiber axis and the axis of revolution of the plug end portion is commonly referred to as the exit angle of the plug.
Control of the exit angle as well as that of lateral offset is essential for achieving low loss connections and high yields in optical fiber connector manufacture. The control of these parameters insures that when two plugs are disposed in an alignment sleeve, not only will the end faces just touch, but the fiber axes will be substantially coaxial.
Seemingly, the prior art is devoid of a simple solution to the problem of providing production plugs at a relatively high yield for biconic connectors which may be used for multi or single mode lightguide fibers. Each production plug must be such that a centroid of the core of an optical fiber terminated therein in an end face of the plug is coincident with the axis of revolution of the truncated, conically shaped surface of the plug and such that the centroidal axis of the end portion of the fiber core in the plug is coaxial with the axis of revolution of the end portion of the plug. Desirably, the solution does not require additional elements or time in the connection procedures, but instead involves an automatic reconfiguration of molded plugs to achieve precision without the need of a skilled machinist. What is needed are methods and apparatus for measuring the exit angle and for correcting the exit angle of a plug by aligning the fiber core axis with the axis of rotation of a turntable on which the plug is held and reconfiguring a new end portion having an axis of revolution which is coaxial with the fiber core axis.